There’s fuel in slime. That was part of the message given by Michael Pacheco, director of the National Bioenergy Center, at a recent conference. Algae as a source of biofuels, according to Pacheco, has enormous potential because it is much more productive than other sources of biomass-up to 50 times more productive than traditional crops.
Speaking at the University of Houston’s Global Energy Management Institute’s Conference, “The Future of Gulf Coast Refining,” on Nov. 3, Pacecho also reviewed potential biomass sources, processes, and products. Although it represents about 3% of the US energy supply, biomass has much potential for supplying transportation fuels.
It is even possible to feed biomass products to conventional refining process units, such as an FCC or diesel hydrotreater, according to another study.
The National Bioenergy Center, which is part of the National Renewable Energy Laboratory (NREL), considers biomass feedstocks to include trees, grasses, agricultural crops, residues, animal wastes, municipal solid waste, food oils, and algae. Processes to convert these to useable products include enzymatic fermentation, gas-liquid fermentation, acid hydrolysis/fermentation, gasification, combustion, cofiring, and transesterification.
The biofuels products, besides ethanol and biodiesel, include green diesel (fats, waste oils, or virgin oils converted to low-sulfur diesel in refining units), other fermentation products, pyrolysis liquids, and algae-derived fuels.
Biomass basics
Biomass can be divided into edible and nonedible constituents. The edible constituents include starch from corn, which is the basis for existing biorefineries (ethanol); oils from corn or soybeans, which are the basis for biodiesel; and proteins from corn or soybean meal.
“These are things that we don’t view in the long run as being the core of the bioenergy industry...these materials are very limited in their supply,” Pacheco said. “The materials that we focus our research on are the lignin, hemicellulose, and cellulose (nonedible constituents).”
The reason for the shift in research is that the nonedible constituents represent a much larger resource than the edible constituents. According to Pacheco, corn (edible) converted to ethanol has the potential to displace 10% of US gasoline supplies. Soybeans, fats, and greases (also edible), can potentially displace 5-10% of US diesel in the form of biodiesel. On the other hand, more than 1 billion tons/year of lignocellulosic biomass (nonedible) could be available in the US, potentially displacing 50-70% of gasoline use.
Refinery feedstocks?
NREL, in conjunction with UOP LLC, has studied the possibility of using oils, fats, and greases as petroleum refinery feedstocks. The results, which were presented at the First International Biorefinery Workshop July 20-21, 2005, in Washington, DC, showed that oils and greases fed to an FCC unit or a distillate hydrotreater can yield products similar to or better than those from a crude oil feed.
The “green diesel” produced entails lower capital costs than a biodiesel plant and has excellent fuel properties. The unit diesel yield is nearly 90% of the feed, the cetane number is 80-100, and the sulfur content is less than 10 ppm.
“The quality of the diesel is just phenomenal,” Pacheco said. “This is a fascinating technology that represents a better fit with the asset base that [refiners] have.”
Algae
NREL is working on algae technologies for creating nonedible lipids. Algae can store carbon in two ways: as a lipid or as a carbohydrate.
“What we are working on is engineering algae strains that produce a very high yield, up to 60%, of their weight as lipids,” Pacheco said. “Lipids are a very highly energy-dense material, the same as diesel fuel particularly when we crack it.
“The other reason we are looking at algae is because it integrates very well with carbon capture. When you take an enriched carbon dioxide stream, whether it’s a flue gas or a concentrated stream for sequestration, you can bubble that CO2 through the solution that you are growing the algae in...you dissolve that CO2 in the growth media and can kick up the growth rate of the algae by about a factor of 50.”
For soybeans, the fuel yield is about 300 gal/acre; for corn the yield is 800-900 gal/acre. For algae, 1 acre can yield 10,000-20,000 gal of fuel, according to Pacheco.
“With the research we are doing, we’ve estimated that about 25 million acres would produce all the oil that the US uses,” Pacheco said. “This is a game-changing technology if we can make it work.”
